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Why days on Earth are getting longer

Researchers used rock sediments to analyse the Earth's relationship to the moon 1.4 billion years ago.

They discovered the moon's effect on the Earth meant a day lasted just over 18 hours back then.

This is partly because the moon used to be much closer, changing the way the Earth spins on its axis, and is moving away from the planet.

"As the moon moves away, the Earth is like a spinning figure skater who slows down as they stretch their arms out," explained Professor Stephen Meyers, professor of geoscience at the University of Wisconsin-Madison.

The moon is currently moving away from the Earth at a rate of 3.82 centimetres a year - a speed which poses tough questions for scientists.

Assuming that rate of movement has been constant, the moon would have been so close to the planet 1.5 billion years ago that the Earth's gravity should have ripped it apart.

However, scientists have established that the moon is 4.5bn years old.

Professor Meyers, a co-author of the study which has been published this week in the Proceedings of the National Academy of Sciences, said the team used a new statistical method which allows scientists to investigate the geologic past of the Earth and explore ancient climate change.

"One of our ambitions was to use astrochronology to tell time in the most distant past, to develop very ancient geological time scales," he said.

"We want to be able to study rocks that are billions of years old in a way that is comparable to how we study modern geologic processes."

A team of researchers at the Lamont-Doherty Earth Observatory at Columbia University used a rock formation in Arizona to identify the regularity of Earth's orbital fluctuations.

Another team in New Zealand worked with Professor Meyers to identify how changes in Earth's orbit and rotation on its axis have affected cycles of evolution and extinction of marine organisms called graptoloids, going back 450 million years.

"The geologic record is an astronomical observatory for the early solar system," said Prof Meyers.

"We are looking at its pulsing rhythm, preserved in the rock and the history of life."